In wireless communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the wireless communication network is deployed.
The growing demands on communication networks to support data applications at higher throughputs and spectral efficiencies have driven the need to develop Orthogonal Frequency Division Multiplexing (OFDM) based 4th generation (4G) networks including the 3rd generation partnership program (3GPP) Long Term Evolution (LTE) telecommunications standards. A key objective with respect to deployment of OFDM 4G networks is to utilize a frequency re-use of one (denoted by N=1), or as close to N=1 re-use as is practical for the particular communication network at hand. A frequency re-use of N=1 implies that the network nodes (such as the evolved node B, eNB, in LTE) in the cells transmit on all available time-frequency resources blocks (RBs) simultaneously. The need for higher throughputs in 4G networks, especially near the cell edge, combined with the constraint on the uplink link budget may necessitate the need for smaller cell sizes than is typically deployed for present 2nd generation (2G) and 3rd generation (3G) cellular radio communication systems. The addition of smaller cells can be deployed in a traditional homogenous cell splitting approach or in a more ad hoc heterogeneous approach in which pico cells or relay nodes, or integrated WiFi nodes are overlaid on or an extension of an existing macro cellular network. For both a homogeneous and heterogeneous approach, the resulting interference limited system for N=1 deployment may not achieve the full potential capacity that the LTE standard can support without the implementation at the network node and wireless device of one or more viable interference mitigation and or cancellation techniques.
Interference cancellation and mitigation techniques have been investigated and deployed with varying degrees of success in terrestrial mobile networks for over 20 years. Traditional approaches to interference mitigation between transmitted signals have focused on either ensuring orthogonality between transmitted signals in time and/or frequency as well as spatially, or by actively removing and cancelling interfering signals from the desired signal if orthogonality between the desired signal and potential interferers cannot be achieved. In early 2G cellular systems such orthogonality was achieved primarily through static pre-planned allocations of radio resources. 3G systems introduced interference cancellation techniques based mostly on a combination of blind information gathering at a network node such as spectrum usage monitoring and coarse exchange of interference indicators such as the Rise over Thermal (RoT) indicator employed in the 3GPP2 1×EV-DO standard. Typically, interfering signals have been estimated using blind detection and their estimates subtracted from the desired signals.
From a link perspective the downlink (DL) allows for a more tractable analysis since if the desired wireless device location is known, the distances to all potential interfering network nodes can be determined based on the network geometry and hence a probabilistic based estimate of the signal-to-interference-plus-noise ratio (SINR) can be calculated based on channel fading conditions for the desired signal and the interfering signals. In addition to additive white Gaussian noise (AWGN,) both the desired signal and interfering signals will experience shadowing which typically is log-normally distributed. Analysis of the uplink (UL) interference requires knowledge of not only the location of the desired wireless device under consideration, but also the relative locations of all potential interfering wireless devices, for which both the locations of the interfering terminals, the number of potential terminals as well as their spatial velocities will behave as random variables.
In cellular networks it is a well known problem that in medium to heavy loading, the network becomes interference limited which can result in negative signal-to-interference-plus-noise (SINR) ratios, particularly for cell edge users.
The challenge with deploying a static N=1 frequency re-use OFDM system in an interference limited environment is that for a fully loaded deployment, significant regions of coverage will experience negative SINR levels resulting in gaps in the deployed coverage, irrespective of the inter-cell distance. In an interference limited system it is not uncommon for, on the order of 15%, of users to experience negative SINR, with some users experiencing negative SINR levels of −10 to −15 dB. It should be noted that in a fully loaded interference limited cellular deployment, the severity of the SINR degradation will be dependent on the average path loss exponent. For a cellular deployment with a fixed inter-cell distance, high path loss propagation environments with path loss exponents up to a 5th or 6th order will experience less overall interference than deployments with lower path loss exponents, since potential interfering signals from neighbouring cells will be more greatly attenuated in the former case. Even though there will be significant SINR variation depending on the propagation environment, in order to robustly deploy an LTE OFDM system one will have to mitigate the inevitable negative SINR coverage regions that will exist.
Coordinated Multipoint (CoMP) transmissions from multiple eNBs to a UE on the DL or from one wireless device to multiple eNBs on the UL is an approach that can be statically or adaptively employed in heterogeneous cellular network deployments to improve the overall SINR levels, particularly for cell edge users. CoMP implementations can be categorized into joint processing solutions or coordinated solutions. Coordinated solutions can be further categorized into coordinated beamforming or coordinated scheduling. The gain in SINR that can be achieved with UL CoMP is typically at a cost of complexity and an increase in required backhaul signalling, particularly for joint UL processing options.
Both the LTE and LTE-Advanced air interfaces support features that mitigate interference. However, most of the straightforward solutions that exploit these interference mitigation capabilities consume excessive backhaul bandwidths and require a significant use of signal processing resources.
Previous approaches to reduce the backhaul overhead for UL CoMP solutions employed sending soft probabilistic metrics such as log-likelihood ration (LLR) estimates, only for cooperating cells that have a signal above a given SINR threshold. One of the disadvantages of this approach is that it still requires the full sampling of the desired signal at each of the cooperating network nodes in the CoMP solution and can still require the use of large backhaul capacities even when only sending LLR ratio information for cooperating network nodes above a predefined SINR threshold.
Due to evolution of network nodes, a number of standards will evolve to maximize commonality to achieve a multi-standard architecture, given that a network node may support multiple standards. Hence, HSPA which traditionally have network nodes connected to an RNC may also have fiber optic cables connected between the network nodes as well. In the context of UL CoMP, if the fiber optic cables between network nodes are to be shared by both HSPA and LTE architectures, the limit of the capacity of these backhaul connections will be even more severe.
Hence, there is still a need for efficient communications between network nodes.